[PATCH] Flag to enable IEEE-754 friendly FP optimizations

[Sergey Dmitrouk]

Hello Mehdi,

It will require passing the flag along with TargetLibraryInfo, but it
should be possible.

I'm not sure I completely understand what the comment says.  I'd turn it
into something like

    if (KeepExceptions &&
        (Op->getValueAPF().isNaN() || Op->getValueAPF().isInfinity()))
      return nullptr;

Which means that if we don't care about floating-point exceptions at
run-time, we also don't care if host libm raises exceptions properly.
Is this correct?  If yes, there is a drawback that default behaviour
will change for "some host libms", which is undesirable.  Please correct
me if I misunderstood you.

Regards,
Sergey

On Mon, Jan 12, 2015 at 09:43:09AM -0800, Mehdi Amini wrote:
> Hi,
>
> Oh great some flags to help with floating point corner cases :)
>
> A question: lib/Analysis/ConstantFolding.cpp contains this code that prevent folding NaN/Inf in InstCombine:
>
>       /// We only fold functions with finite arguments. Folding NaN and inf is
>       /// likely to be aborted with an exception anyway, and some host libms
>       /// have known errors raising exceptions.
>       if (Op->getValueAPF().isNaN() || Op->getValueAPF().isInfinity())
>         return nullptr;
>
> Can this be tied to KeepExceptions?
>
> Thanks,
>
> Mehdi
>
> > On Jan 12, 2015, at 12:21 AM, Sergey Dmitrouk <sdmitrouk at accesssoftek.com> wrote:
> >
> > Ping.
> >
> > On Mon, Dec 22, 2014 at 11:36:10AM +0200, Sergey Dmitrouk wrote:
> >> Hello Hal,
> >>
> >> it took me some time to change the way it works, but now new changes also
> >> cover some cases missed in the previous version.
> >>
> >> After thinking a bit more I introduced two separate flags for exceptions
> >> and rounding, as you suggested.  At the moment rounding flag just
> >> disables folding, I'm not sure how to actually check that result of
> >> operation doesn't depend on rounding mode.  This prevents early
> >> optimization for something like "1.0 + 2.0", so might need adjustments
> >> if there is a way to check whether rounding is important for particular
> >> operation.
> >>
> >> Using fast-math flags instead of function attributes didn't remove that
> >> much flags as they are not used in code related to constant expression
> >> folding.  In fact, I had to add one more "Strict" argument to be able to
> >> prevent conversion to constant expressions, which results in losing
> >> fast-math flags.
> >>
> >> Please find updated set of patches in the attachment.
> >>
> >> Thanks,
> >> Sergey
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